The present invention relates generally improvements in capacitors. The invention has particular utility in connection with electrolytic capacitors comprising anodes formed from tantalum, for use in high voltage applications, and will be described in connection with such utility, although other utilities, e.g., for use as battery anodes, are contemplated.
Tantalum-based electrolytic capacitors have found increasing use in electronics. The combination of small package size, insensitivity to operating temperature, and excellent reliability have made them the choice over ceramic multilayer and aluminum foil-based capacitors for many applications. As the state of the art in electronics continues to progress, demand has grown for more efficient tantalum electrolytic capacitors.
An electrolytic capacitor has three basic components: an anode, a cathode, and an electrolyte. Heretofore, electrolytic tantalum anodes have primarily been fabricated using fine particle tantalum powder. The powder is pressed into a green compact (20 to 50 percent dense) and is sintered under vacuum at a temperature of 1500°-2000° C. for 15-30 minutes to form a porous, mechanically robust body in which the tantalum is electrically continuous. The sintering process is, in some cases, relied upon to attach a lead wire to the compact. In these cases, the lead is inserted into the green compact prior to sintering. If the lead is not attached in this manner, it may be welded into place immediately following sintering of the compact. An important ancillary benefit of the sintering operation is purification of the tantalum particle surfaces; impurities, such as oxygen, are driven off.
After sintering, the compact is anodized to form the dielectric tantalum pentoxide (Ta2 O5) on the exposed surfaces. The porous regions of the anodized compact are then infiltrated with a conductive electrolyte. The electrolyte may be of the “solid” or “wet” type. Depending upon the application, a wet electrolytic capacitor may offer advantages over a solid electrolytic capacitor. Wet electrolytic capacitors tend to be larger than solid electrolytic capacitors and can offer higher capacitance as a result. This is desirable, since what is needed in many modern applications are capacitors with high energy densities.
State of the art tantalum powder is produced by the sodium reduction process of K2TaF7. Improvements in the process have resulted in commercially available powders capable of yielding a specific capacitance of over 50,000 CV/g. Better control of input tantalum particle size, reaction temperature, and other variables has led to the improvements in specific capacitance. A key advance was the introduction of doping agents that enabled the production of very high specific capacitance powders. The doping agents serve to prevent surface loss during sintering. Typical additives are nitrogen, oxygen, sulfur, and phosphorus compounds in the range from 50 to 500 ppm. While select dopants are beneficial, it is important to limit other contamination, which can weaken the dielectric film or even prevent the formation of a continuous Ta2O5 layer that could lead to premature breakdown of the dielectric film and loss of capacitance.
Higher capacitance tantalum particles have been obtained by ball milling powders. Ball milling turns the roughly spherical powder particles into flakes. The benefit is that the flakes can have a higher specific capacitance at higher formation voltage than powder particles. This translates into greater volumetric efficiency for the flakes when they are formed into anodes. Aspecting tantalum particles by ball milling and other processes aimed at improving powder performance, while effective, has practical drawbacks, including increased manufacturing costs and a marked decrease in product yield. Currently, a premium of 2-3 times is charged for the very highest capacitance flakes over standard powder product.
The very fine tantalum powders commercially available today have several serious problems with respect to anode fabrication. One significant problem in particular is a sensitivity to surface area loss during sintering. Ideal sintering conditions are high temperatures and short times. A higher temperature serves to purify the tantalum surface and provide a mechanically strong compact. Capacitors having lower equivalent series resistance (ESR) and equivalent series inductance (ESL) can be fabricated if higher sintering temperatures are employed. Unfortunately, the fine particles of high capacitance powders and flakes lose surface area at temperatures over 1500° C. A loss of surface area results in lower capacitance, reducing the benefit of using the higher specific capacitance powder. The capacitor manufacturer must balance sintering temperature, mechanical properties, and ESR and ESL levels in order to maximize capacitor performance.
Fine powders and flakes are also sensitive to forming voltage during anodization. The anodization process consumes some of the metallic tantalum to form the dielectric layer. As the forming voltage increases, more of the tantalum is consumed, resulting in a loss of capacitance. As the powder becomes finer, this problem becomes increasingly serious.
In practice today, high surface area powders used in capacitor anodes are sintered at low temperatures (below 1500° C.) and are anodized at voltages below 150 volts. Most of these capacitors are restricted to operating voltages below 100 volts.
Also, when tantalum powders are formed into a porous anode body and then sintered for use in an electrolytic capacitor, it is known that the resultant anode capacitance is proportional to the specific surface area of the sintered porous body. The greater the specific surface area after sintering, the greater the anode capacitance (μFV/g) is. Since the anode capacitance (μFV/g) of a tantalum pellet is a function of the specific surface area of the sintered powder, one way to achieve a greater net surface area is by increasing the quantity (grams) of powder per pellet. However, with this approach cost and size increase considerably. Consequently, cost and size considerations dictate that tantalum powder development focus on means to increase the specific surface area of the powder itself.
One commonly used tantalum powder having relatively large particles is commercially available from H. C. Starck under the designation QR-3. This so called EB melt-type tantalum powder permits anodes to be made with relatively larger pore structures. However, the relatively low specific surface area of these large particle size powders does not result in anodes of high capacitance per unit volume. Another commonly used material is available from H. C. Starck as sodium reduced tantalum powder under the designation NH-175. Because of its relatively higher surface area, this material is known to produce anodes with higher capacitance than QR-3 powders. However, because of its smaller feature size and broad particle size distribution, NH-175 powders are also known to produce anodes with smaller pore structures. The smaller pore structure makes internal cooling of anode pellets during anodization more difficult, and thus limit an formation voltages that these anodes can achieve.
Purity of the powder is another important consideration. Metallic and non-metallic contamination tends to degrade the dielectric oxide film in tantalum capacitors. While high sintering temperatures serve to remove some volatile contaminants, not all may be removed sufficiently, resulting in sites having high DC leakage. High DC leakage is known to contribute to premature electrical failures, particularly in high voltage applications. Further, high sintering temperatures tend to shrink the porous anode body, thereby reducing its net specific surface area and thus the capacitance of the resulting capacitor. Therefore, minimizing loss of specific surface area under sintering conditions, i.e., shrinkage, is necessary in order to produce high μFV/g tantalum capacitors.
In my prior U.S. Pat. No. 5,034,857, I disclose an approach to the production of very fine valve metal filaments, preferably tantalum, for forming anodes. The benefits of fine filaments relative to fine powders are higher purity, uniformity of cross section, and ease of dielectric infiltration, while still maintaining high surface area for anodization. The uniformity of cross section results in capacitors with high specific capacitance, lower ESR and ESL, and less sensitivity to forming voltage and sintering temperature as compared to fine powder compacts.
As disclosed in my aforesaid '857 U.S. patent, valve metal filaments, preferably tantalum, are fabricated through the combination of the filaments with a ductile metal so as to form a billet. The second, ductile metal is different from the metal that forms the filaments. The filaments are substantially parallel, and are separated from each other and from the billet surface by the second, ductile metal. The billet is reduced by conventional means—e.g., extrusion and wire drawing—to the point where the filament diameter is in the range of 0.2 to 5.0 microns in diameter. At that point, the second, ductile metal is removed, preferably by leaching in mineral acids, leaving the valve metal filaments intact. The filaments are suitable for use in tantalum capacitor fabrication.
Other patents involving valve metal filaments and fibers, their fabrication, and articles made therefrom include U.S. Pat. No. 3,277,564, (Webber), U.S. Pat. No. 3,379,000 (Webber), U.S. Pat. No. 3,394,213, (Roberts), U.S. Pat. No. 3,567,407 (Yoblin), U.S. Pat. No. 3,698,863 (Roberts), U.S. Pat. No. 3,742,369 (Douglass), U.S. Pat. No. 4,502,884 (Fife), U.S. Pat. No. 5,217,526 (Fife), U.S. Pat. No. 5,306,462 (Fife), U.S. Pat. No. 5,284,531 (Fife), and U.S. Pat. No. 5,245,514 (Fife).
See also my prior U.S. Pat. No. 5,869,196, my prior U.S. Pat. No. 8,858,738, and my U.S. Pat. No. 8,673,025 in which I describe various processes for reducing valve metal filaments or wires in a ductile metal matrix by extrusion and drawing; cutting the extruded filaments into short segments, leaching out the ductile metal so as to leave the short valve metal filaments intact, and forming or casting the valve metal filaments into a thin sheet from a slurry for use in forming anodes and cathodes formed from fine valve metal filaments.
While anodes formed from fine valve metal filaments such as described in my aforesaid '857, '196, '738 and '025 patents provide superior performance to anodes formed from pressed powder, anodes formed from fine metal filaments are more expensive and thus have limited utility to special applications.